The present invention relates generally to fluidized bed boilers, and more particularly to pantleg circulating fluidized bed boilers having multiple fuel feed points.
Fluidized bed reactors are known for use in combustion and non-combustion reactor systems. One of the primary advantages of using fluidized bed reactors as contrasted to fixed bed reactors in combustion reactor systems is that the fluidized bed reactors burn coal efficiently at a relatively low temperature, thereby resulting in minimal nitrogen-oxide production. Furthermore, the high thermal inertia of the bed mass provides for good performance when firing low-volatile fuels such as anthracite, anthracite culm, and petroleum coke. A sorbent material can be added to the reactor in order to control sulfur dioxide emissions. Therefore, it is not necessary to include a stack-gas SO.sub.2 scrubber. The sulfur sorbent also can react with other fuel constituents such as vanadium, reducing down stream corrosion potential.
The two standard types of fluidized bed reactors are bubbling fluidized beds and circulating fluidized beds. In a bubbling fluidized bed (BFB), which is characterized by relatively slow gas velocity and coarse bed-particle size, the conduction/convection heat transfer is to furnace wall tubes and other heating surfaces that may be immersed in the bed. Furthermore, radiation heat transfer occurs above the active bed. In a circulating fluid bed (CFB), which is characterized by high gas velocities and a finer bed-particle size, the bulk of conduction/convection heat transfer is to the combustor wall tubes.
It is known to design a combustion system which uses a pantleg CFB boiler in conjunction with a pair of external fluid bed heat exchangers (FBHEs). When a compact system design is desired, integral FBHEs such as those described in U.S. Pat. No. 4,716,856, the contents of which are incorporated herein by reference, can be used. In a conventional system, one FBHE extends along the front wall of the CFB boiler and the other extends along the rear wall of the CFB boiler at the lower end of the boiler.
In the conventional configuration of a pantleg CFB boiler equipped with integral, external FBHEs, there is a limitation as to the number of gravity fuel feed points which are available. Because the gravity fuel feed points should be approximately six feet above the combustor grate, the location of the FBHEs along the front and rear walls of the boiler precludes the use of fuel feed inlets on the front and rear walls. Typically, the fuel is fed into cyclone seal pots which provide for entry of the fuel along with the recycled solids through inlet ports on the side walls of the boiler. A conventional system of this type includes four seal pots, and therefore a conventional pantleg CFB boiler has only four fuel feed inlets. For small boilers, this number of feed points is sufficient. However, for larger boilers, four fuel feed points may be insufficient to produce the result of high combustion efficiency. In addition to using an unnecessarily large amount of fuel, the use of an insufficient number of fuel feed points will likely result in the generation of undesirable emissions, at least when fuel having a low reactivity is used. It would be advantageous to be able to include additional fuel feed points in large pantleg boilers of this type in order to reduce the theoretical mixing length of the fuel within the boiler.